In order to approximate the appearance of continuous-tone (photographic) images via ink-on-paper printing, the commercial printing industry relies on a process known as halftone printing. In halftone printing, color density gradations are produced by printing patterns of dots or areas of varying sizes, but of the same color density, instead of varying the color density continuously as is done in photographic printing.
There is an important commercial need to obtain a color proof image before a printing press run is made. It is desired that the color proof will accurately represent at least the details and color tone scale of the prints obtained on the printing press. In many cases, it is also desirable that the color proof accurately represent the image quality and halftone pattern of the prints obtained on the printing press. In the sequence of operations necessary to produce an ink-printed, full-color picture, a proof is also required to check the accuracy of the color separation data from which the final three or more printing plates or cylinders are made. Methods for preparing such proofs by means of laser-induced transfer of colorant from suitable donor elements are well known in the art.
In U.S. Pat. No. 5,126,760, a process is described for producing a direct digital, halftone color proof of an original image on a dye-receiving element. The proof can then be used to represent a printed color image obtained from a printing press. The process described therein comprises:
a) generating a set of electrical signals which is representative of the shape and color scale of an original image; PA1 b) contacting a dye-donor element comprising a support having thereon a dye layer and an infrared-absorbing material with a first dye-receiving element comprising a support having thereon a polymeric, dye image-receiving layer; PA1 c) using the signals to imagewise-heat by means of a diode laser the dye-donor element, thereby transferring a dye image to the first dye-receiving element; and PA1 d) retransferring the dye image to a second dye image-receiving element which has the same substrate as the printed color image. PA1 (1) a propellant layer comprising a gas-producing polymer having an infrared-absorbing material associated therewith, the gas-producing polymer being capable of forming a gas upon heating by the laser, and PA1 (2) a colorant transfer layer comprising a colorant dispersed in a binder, at least 50% by weight of the binder being a monomeric or oligomeric resin having a molecular weight of less than about 4,000 and the balance being a polymeric resin having a molecular weight of from about 15,000 to about 100,000;
In the above process, multiple dye-donors are used to obtain a range of colors in the proof. For example, for a full-color proof, four colors: cyan, magenta, yellow and black are normally used.
By using the above process, the image dye is transferred by heating the dye-donor containing the infrared-absorbing material with the diode laser to volatilize the dye, the diode laser beam being modulated by the set of signals which is representative of the shape and color of the original image, so that the dye is heated to cause volatilization only in those areas in which its presence is required on the dye-receiving layer to reconstruct the original image.
U.S. Pat. No. 5,278,023 discloses propellant-containing thermal transfer donor elements which are used to mass transfer colorants to a receiver element. However, there is a problem with this element in that either poor resolution or poor adhesion to a receiving element is obtained.
U.S. Pat. No. 5,089,372 discloses a process whereby colorant layer transfer by melt or wax transfer is assisted by gaseous nitrogen produced in an underlying layer. However, a wax transfer process necessitates that the donor element be in contact with the receiver element so that this process is prone to image artifacts due to dust and dirt being trapped between the donor and receiver element.
It is an object of this invention to provide a process for producing a color proof that has better resolution and adhesion than the prior art elements. It is another object of this invention to provide a process for producing a color proof wherein the donor element is separated from the receiver element by an air gap so that image artifacts in the final print due to dust and dirt are minimized.